This invention relates to the field of optical and magneto-optical recording. More particularly, it relates to improvements in optical head apparatus for irradiating a moving recording element with a focused beam of radiation.
Virtually all optical and magneto-optical recording systems include some sort of mechanical servo for maintaining an optical write/read beam in sharp focus on a rapidly moving recording element (e.g., a spinning optical disk). Such focus servo generally functions to continuously sense changes in the state of focus, as occurs when the recording surface moves, in a random fashion, toward and away from a nominal focus position relative to a focusing lens, and to continuously reposition the focusing lens to maintain a best focus condition. Due to the high numerical aperture (N.A.) of the focusing lens and the wavelength of the read/write beam, the focus servo must be sufficiently sensitive to maintain the lens-to-recording layer spacing to within .+-.1 micron.
In U.S. Pat. No. 4,725,721 issued to Nakamura et al, there is disclosed a self-focusing optical head which substantially relaxes the sensitivity requirements of the focus servo in an optical recording system. According to this disclosure, a beam of radiation emitted by a semiconductor laser is brought to focus on an optical recording element by a lens which exhibits substantial chromatic aberration. Radiation reflected by the recording element is intentionally coupled back to the laser cavity via the focusing lens and, depending on the spacing between the lens and recording element, the laser oscillates at one of a plurality of discrete longitudinal modes (wavelengths) within a certain wavelength range. Here, the laser and the recording element constitute an external resonator, which assures that the most powerful oscillation occurs at a wavelength most efficiently returned to the laser's internal cavity. By virtue of this arrangement, the focused spot follows for itself the movement of the disk over a displacement range of .+-.12 microns by self-adjusting the wavelength of the laser source. As a result, the focus servo need only be capable of coarsely positioning the focusing lens to within .+-.12 microns of a best focus position.
While the self-focusing optical head disclosed by Nakamura et al may reduce the sensitivity requirements of the focusing servo by an order of magnitude, the laser wavelength changes on which the self-focusing effect relies can degrade the servo mechanism performance. Longitudinal mode changes in a semiconductor laser occur suddenly, on a sub-nanosecond timescale. Such wavelength changes alter the radiation distribution at the focus detector and give rise to a focus error signal (S-curve) having sudden changes in level. FIG. 1 illustrates how the focus error signal appears if the laser wavelength changes by .+-.2 modes near the point of best focus (i.e., zero error signal). Overall, the slope of the curve appears to be flattened near focus, because each wavelength change tends to improve focus. But, as shown, the detailed shape of the curve is discontinuous, with sharp changes. This kind of distortion in the error signal acts to degrade the servo performance in two ways. Firstly, the steep edges can amplify undesired high frequency resonances in the electromechanical system. Secondly, and especially if these edges are electronically filtered out, the slope of the error signal near best focus will be very shallow and imprecisely define the point of best focus. The lack of detector linearity produced by these sudden wavelength shifts allows the mechanical system to oscillate at small amplitudes without creating a substantial direct error signal, thereby making the system unstable to any larger perturbations.